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Etude volcano-structurale du volcan Nemrut (Anatolie

de l’Est, Turquie) et risques naturels associés

Inan Ulusoy

To cite this version:

Inan Ulusoy. Etude volcano-structurale du volcan Nemrut (Anatolie de l’Est, Turquie) et risques naturels associés. Volcanologie. Université Blaise Pascal - Clermont-Ferrand II, 2008. Français. �NNT : 2008CLF21855�. �tel-00730602�

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Numéro d’Ordre: D.U. 1855

UNIVERSITE BLAISE PASCAL – CLERMONT FERRAND II

U.F.R. Sciences et Technologies

ECOLE DOCTORALE DES SCIENCES FONDAMENTALES

N° 577

THESE

présentée pour obtenir le grade de

DOCTEUR D’UNIVERSITE

Spécialité : Volcanologie

Par

ULUSOY İnan

Master

Etude volcano-structurale du volcan Nemrut

(Anatolie de l’Est, Turquie) et risques naturels

associés

Soutenue publiquement le 18 Septembre 2008, devant la commission d’examen

Président : LENAT Jean-François Université Blaise Pascal - Clermont-Ferrand

Examinateur : YÜRÜR Tekin Université Hacettepe - Ankara

Rapporteur : LESAGE Philippe Université de Savoie - Le Bourget du Lac

Rapporteur : BOZKURT Erdin Univ. Technique de Moyen Orient - Ankara

Directeur de thèse : AYDAR Erkan Université Hacettepe - Ankara

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NEMRUT VOLKANI’NIN YAPISAL – VOLKANOLOJİK

İNCELENMESİ VE DOĞAL RİSK POTANSİYELİNİN

BELİRLENMESİ

VOLCANO – TECTONIC INVESTIGATION OF NEMRUT

VOLCANO AND DETERMINATION OF ITS NATURAL RISK

POTENTIAL

İNAN ULUSOY

Hacettepe Üniversitesi

Lisansüstü Eğitim – Öğretim ve Sınav Yönetmeliğinin JEOLOJİ Mühendisliği Anabilim Dalı İçin Öngördüğü

DOKTORA TEZİ olarak hazırlanmıştır.

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Résumé

Le volcan actif Mont Nemrut, situé à l’ouest du lac Van, est l'un des volcans les plus importants d’Anatolie orientale. Il possède une caldeira sommitale de 8.5×7 kilomètres de diamètre. L'activité volcanique du Nemrut a commencé il y a ~1 Ma et s’est poursuivie jusque dans les périodes historiques. Les éruptions les plus récentes ont été signalées en 1441, 1597 et 1692 A.D. Parmi les volcans anatoliens orientaux; le Nemrut est le volcan le plus dangereux, compte tenu de sa proximité avec des sites urbanisés environnants ; il menace directement 135.000 habitants. Les manifestations actuelles de l’activité volcanique sont représentées par une activité hydrothermale et fumerollienne au sein de la caldeira.

L'évolution structurale du volcan se subdivise en deux stades principaux (pré-caldeira et post-(pré-caldeira) séparés par l'effondrement catastrophique de la (pré-caldeira. Les produits de l’activité anté-caldeira sont majoritairement représentés par des écoulements et des dômes de lave felsiques. Les séries ignimbritiques du Nemrut et de Kantaşı, manifestations majeures de l’activité de la caldeira, sont constituées d’unités pliniennes et d’écoulements ignimbritiques. L'activité post-caldeira est représentée par une activité phréatomagmatique explosive et une activité effusive basaltique-rhyolitique bimodale, concentrées au sein de la caldeira et au niveau de la zone de rift récente du Nemrut, sur le flanc nord.

L’analyse des données multisources (études de polarisation spontanée, modèles numériques de terrain et bathymétrie ainsi que leurs produits dérivés, images Landsat et ASTER) a permis de caractériser la structure de la caldeira du Nemrut et les circulations hydrothermales associées. La synthèse de ces approches pluri-thématiques et des interprétations correspondantes permet de proposer que la caldeira est constituée de trois blocs principaux, conséquence des processus de fragmentation générés lors des phases d’effondrement. Les frontières délimitant ces blocs et la frontière structurale principale de la caldeira contrôlent les principales activités hydrothermales intra-caldeira.

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Le régime tectonique régional de compression-extension existant au Pliocène est structuralement enraciné et est responsable du déclenchement du volcanisme du Mont Nemrut. Le jeu de systèmes décrochants surimposés aux structures pré-existantes a provoqué l’apparition d’une zone de faiblesse localisée au sein de laquelle le système volcanique du Nemrut s’est préférentiellement mis en place.

La surveillance de l’activité du volcan Nemrut a été initiée avec l’installation d’un ensemble de trois sismomètres, ce qui constitue le premier réseau de surveillance sismo-volcanique sur un volcan en Turquie. Les données temps réel sont acquises, traitées et archivées depuis octobre 2003. L’interprétation des signaux volcaniques acquis dans le cadre de cette surveillance sismologique, couplée aux résultats de l’étude du système hydrothermal, confortent clairement l’existence d’une chambre magmatique active localisée aux environs de 4-5 kilomètres de profondeur. La surveillance à long terme de ce volcan potentiellement actif est essentielle pour la prévention des risques associés et fournira de plus une base de données essentielle pour une meilleure connaissance et compréhension du mode de fonctionnement de ce volcan.

Mots Clés: Nemrut, Lac Van, Anatolie de l’Est, Turquie, Polarisation spontanée, ASTER, Imagerie Infrarouge thermique, système hydrothermal, Surveillance Sismologique, Collision Continentale, Extension, Ignimbrites.

Directeur de thèse : Prof. Dr. Erkan AYDAR, Hacettepe Üniversitesi Jeoloji Mühendisliği Bölümü, Mineraloji – Petrografi Anabilim Dalı.

Co-directeur de thèse : Dr. Philippe LABAZUY, Blaise-Pascal Univ., Laboratoire de Magmas et Volcans.

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Öz

Van gölünün batısında yeralan Aktif Nemrut volkanı, Doğu Anadolu’da yeralan volkanların en önemlilerinden biridir. Zirvesinde 8.5x7 km çapında bir kaldera yeralmaktadır. Nemrut dağının volkanik faaliyeti ~1 My önce başlamış ve tarihsel çağlara kadar devam etmiştir. Volkanın en genç patlamaları M.S. 1441, 1597 ve 1692’de gerçekleşmiştir. Doğu Anadolu’da yeralan volkanlar arasında Nemrut dağı, çevresi için en tehlikeli volkandır ve etrafında yaşayan 135.000 kişiyi tehdit etmektedir. Güncel volkanik faaliyet, kaldera içindeki hidrotermal faaliyet ve buhar çıkışları ile belirgindir.

Volkanın yapısal gelişimi, katastrofik kaldera çökmesi ile ayrılan iki ana evrede (kaldera öncesi ve kaldera sonrası) incelenmiştir. Kaldera öncesi ürünler esas olarak felsik lav akıntıları ve domlardan oluşmaktadır. Pliniyen üniteler ve ignimbirit akıntılarından oluşan Nemrut ve Kantaşı ignimbirit serileri kaldera oluşturan faaliyeti temsil etmektedir. Kaldera sonrası faaliyet ise kaldera içindeki patlayıcı hidrovolkanik ve riyolitik lav akıntıları/domları ile Nemrut rift zonundaki bimodal bazaltik-riyolitik efüzif faaliyet ile temsil edilmektedir.

Nemrut kalderası içindeki hidrotermal akışkan hareketleri ve kaldera’nın yapısallığı, doğal-potansiyel ölçümleri, batimetri verisi, SAM (Sayısal Arazi Modeli) türevleri ve Landsat - ASTER uydu görüntüleri yardımıyla açığa çıkarılmıştır. Kaldera’nın üç ana bloktan oluştuğu ve parçalı tipte çökme ile oluştuğu düşünülmektedir. Bu blokların arasındaki yapısal sınırlar ve ana kaldera sınır fayı kaldera-içi hidrotermal faaliyeti kontrol etmektedir.

Pliyosen’de sıkışma-açılma tektonik rejiminin başlaması Nemrut volkanizmasını tetiklemiş ve yapısal anlamda köklerini oluşturmuştur. Daha önceden var olan yapılara doğrultu-atımı bileşeninin dahil olması, Nemrut volkanının oluşumuna olanak tanıyan yerel açılmalara izin vermiştir.

Nemrut volkanının gözlemlenmesi amacıyla üç adet sismometre ile yerel bir sismik ağ kurulmuştur. Bu ağ, Türkiye’de bir volkanın etrafına kurulan ilk sismik volkan gözlem ağıdır ve 2003 yılından beri gerçek-zamanlı veri sağlamaktadır.

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Hidrotermal durum ve gözlenen volkanik kökenli sinyaller, 4-5 km derinlikte bir aktif magma odasına işaret etmektedir. Sessiz bir dönemde olan bu aktif volkanın uzun süreli gözlemlenmesi elzemdir ve volkan hakkındaki bilgilerimizi artıracaktır.

Anahtar Kelimeler: Turkiye, Doğu Anadolu, Nemrut, Doğal-potansiyel, hidrotermal sistem, ASTER, Gece termal kızılötesi, Sismik gözlem, Kıtasal çarpışma, Açılma, İgnimbiritler, Van Gölü.

Danışman: Prof. Dr. Erkan AYDAR, Hacettepe Üniversitesi Jeoloji Mühendisliği Bölümü, Mineraloji – Petrografi Anabilim Dalı.

Eş Danışman: Dr. Philippe LABAZUY, Blaise-Pascal Univ., Laboratoire de Magmas et Volcans.

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Abstract

Active Mt. Nemrut volcano, situated at the west of Lake Van, is one of the most important volcanoes of the Eastern Anatolia. It has a summit caldera with 8.5×7 km diameter. Volcanic activity of Mt. Nemrut started ~1 Ma ago and has continued in historical times. The most recent eruptions of the volcano were in 1441, 1597 and 1692 A.D. Amongst the Eastern Anatolian volcanoes; Mt. Nemrut is the most hazardous volcano for its vicinity, threatening 135,000 habitants living nearby. The present volcanic activities are represented by hydrothermal and fumarolic output within the caldera.

Structural evolution of the volcano is mainly investigated in two stages (pre-caldera and post-(pre-caldera) separated by catastrophic (pre-caldera collapse. Pre-(pre-caldera products are dominated by felsic lava flows and domes. Nemrut and Kantaşı ignimbrite series represent the caldera forming activity, of which sequences are comprised of plinian units and ignimbrite flows. Post-caldera activity is represented by bimodal basaltic - rhyolitic effusive and explosive hydrovolcanic activity concentrated in the caldera and on Nemrut rift zone.

Hydrothermal fluid circulation paths in Nemrut caldera and the structure of the caldera have been revealed using self-potential surveys, bathymetry data, derivatives of DEMs, Landsat and ASTER images. It is proposed that the caldera consists of three main blocks and has collapsed in a piecemeal manner. Boundaries delimiting these blocks and the main structural boundary of the caldera control the intra-caldera hydrothermal activities.

Initiation of compressional-extensional tectonic regime in Pliocene structurally rooted and triggered the volcanism of Mt. Nemrut. Addition of strike-slip component to the pre-existing structures has led localized extensions where Nemrut volcanic system has been preferentially emplaced.

To monitor the Nemrut volcano, three seismometers were installed. This is the first volcano-seismic monitoring network around a Turkish volcano and real-time data are being collected since October 2003. Hydrothermal signature as well as

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acquired volcanic signals clearly refer to an active magma chamber emplaced around 4-5 km depth. Long term monitoring of this active-quiescent volcano is vital and will yield more knowledge about this volcano.

Keywords: Turkiye, Eastern Anatolia, Nemrut, Self-potential, hydrothermal system, ASTER, night-time thermal infrared, Seismic surveillance, Continental collision, Extension, Ignimbrites, Lake Van.

Advisor: Prof. Dr. Erkan AYDAR, Hacettepe Üniversitesi Jeoloji Mühendisliği Bölümü, Mineraloji – Petrografi Anabilim Dalı.

Co-advisor: Dr. Philippe LABAZUY, Blaise-Pascal Univ., Laboratoire de Magmas et Volcans.

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Acknowledgements

This work was supported by Hacettepe University Scientific Research Foundation research grant (Project No.: 0101602021), by UMR-CNRS 6524 and an integrated PhD grant from the French Ministry of Foreign Affairs. During the period of my PhD study, I have had enormous support and help from the following exceptional people and I am very grateful for all the encouragement that I have received.

My gratitude to my advisor Prof. Dr. Erkan Aydar could never be sufficient in regards to his guidance, support and his faith in me throughout the years; he guided me into new horizons. I will always be proud for having the possibility to work in his group.

I am sincerely grateful to my co-advisor Dr. Philippe Labazuy, for his support and supervision. He always encouraged me for in-depth learning and braced me up. He helped me to built-up the basis of my geophysics and remote-sensing knowledge.

To work with Dr. Evren Çubukçu, Dr. Orkun Ersoy and Dr. Erdal Şen, has always been always challenging, I always enjoyed and benefited to work with them. I will never forget their friendship.

I always felt the caring support from both Prof. Dr. Hasan Bayhan and Prof. Dr. Alain Gourgaud over the years. Without their contribution, it would have been impossible to overcome this project.

I would like to thank Assist. Prof. Dr. Levent Tezcan. I will always consider myself lucky to be able to benefit from his broad knowledge of GIS. I would also like to thank to Prof. Dr. Jean-François Lénat for his courtesy for allowing me to use his code to generate the Ce map.

I would like to express my gratitude to Prof. Dr. Jean-François Lénat, Prof. Dr. Erdin Bozkurt, M. Conf. Dr. Philippe Lesage and Assoc. Prof. Dr. Tekin Yürür for

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honoring me by accepting to be the members of the thesis committee. Their guidance significantly improved this report.

I would like to thank the Governor Generalship and General Command of Gendarmerie of Bitlis City, Tatvan, Güroymak and Ahlat villages; during fieldworks and installation/maintenance process of seismic stations, their help was very valuable. I owe special thanks to Major Oktay Polat for his kind support.

Our driver in fieldtrips, Dursun Kıncal deserves our special thanks here; he is one of the best guides in the region. I am thankful to photographers Adem Sönmez and Oktay Subaşı for letting me use their photographs.

Finally, I am grateful to my parents Filiz and Mahir Ulusoy for all their efforts during my education. It was their support that carried me to this point. I always felt that my wife Ayşe Ulusoy and my sister Gülen Ulusoy have been there for me and kept my moral during hard times. Throughout these years, all the constructive discussions in the family contributed to my academic intelligence.

_________________________________________________________________ “Damla bile değil idim

Göle çevirdiler beni” S. Miskini

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TABLE OF CONTENTS

_________________________________________________________________________

1. INTRODUCTION ... 13

1.1.TECTONICS AND VOLCANISM IN ANATOLIA...15

1.2.NEMRUT CALDERA...22

1.3.PREVIOUS STUDIES...26

1.4.STRUCTURAL APPROACH AND NATURAL RISK OF THE NEMRUT CALDERA...27

1.4.1. Methodology...28

_________________________________________________________________________ 2. GEOLOGY, CALDERA FORMING ERUPTIONS ... 30

2.1.PRE-VOLCANIC BASEMENT...33

2.1.1. Bitlis Metamorphics ...35

2.1.2. Çatak Ophiolites ...35

2.1.3. Tertiary Sediments (Ahlat formation)...35

2.2.VOLCANISM IN THE VICINITY OF MT.NEMRUT...36

2.2.1. Süphan Volcano ...36

2.2.2. Bilican volcano and Kolango dome ...37

2.2.3. İncekaya Tuff Cone ...38

2.3.GEOLOGICAL EVOLUTION OF NEMRUT VOLCANO...39

2.3.1. Pre-caldera activity...43

2.3.2. Caldera forming eruptions (Sub-stage V) ...49

2.3.3. Post-caldera activity ...81

2.4.SYNTHESIS...87

_________________________________________________________________________ 3. HYDROTHERMAL ACTIVITY AND FLUID CIRCULATION...89

3.1.IMAGE BASED RETRIEVAL AND CORRECTION OF ALTITUDE AND ASPECT EFFECTS ON NIGHTTIME TIR IMAGERY...92

3.2.STRUCTURE OF THE NEMRUT CALDERA (EASTERN ANATOLIA,TURKEY) AND ASSOCIATED HYDROTHERMAL FLUID CIRCULATION...109

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_________________________________________________________________________

4. SEISMIC MONITORING OF NEMRUT VOLCANO ...125

4.1.SEISMIC NETWORK...127

4.1.1. Limitations, problems faced and efficiency of the network ...129

4.2DATA PROCESSING...132

4.3.SEISMICITY OF THE VOLCANO...134

4.3.1. Seismicity between October 2003 and October 2005 ...140

4.4.SYNTHESIS...149

_________________________________________________________________________ 5. DISCUSSION AND CONCLUSIONS ...150

5.1.STRUCTURE OF NEMRUT CALDERA...151

5.1.1. Tectonic evolution and initiation of Nemrut volcanism...158

5.1.2. On the formation of Lake Van ...168

5.2.NATURAL RISK POTENTIAL OF NEMRUT VOLCANO...170

5.2.1. Small scale cold lahars...171

5.3.SUGGESTIONS...173

5.4.ONGOING WORKS...174

5.4.1. On the calculation method of Ce Map ...174

5.4.2. Lightweight multi-electrode resistivity cabling system ...181

_________________________________________________________________________ REFERENCES CITED ...183

_________________________________________________________________________ APPENDIX – A ...196

APPENDIX – B ...202

E-APPENDIX – 1: EVOLUTION OF NEMRUT VOLCANO ...215

E-APPENDIX – 2: STRATIGRAPHICAL SECTIONS ...215

E-APPENDIX – 3: “STCORR” AND SHORT USER GUIDE. ...215

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Chapter 01

Introduction

_________________________________________________________________ “Change is the only constant.”

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1. Introduction

Turkey forms one of the most actively deforming regions in the world and has a long history of devastating earthquakes (Bozkurt, 2001). Run of disasters (Kocaeli, M: 7.4; Düzce, M: 7.2) at the end of 1999 aroused interest on active tectonics of Turkey once again. Close location of earthquakes to one of the most crowded cities in the world also attracted interest of the public. Earthquakes, their genesis and results became a popular discussion subject. The popularity of the discussion also banished the subject from its scientific basis which must originally be the root of the following research, construction and reconstruction. After the catastrophic events, large amount of research budget on tectonics (worldwide) were directed into the research under Marmara Sea. Participation and support of local civil authorities to the ongoing research was also polemical. Politicians disregarded the importance of the research needed for future protection. In summary, even the disaster was catastrophic, both funding and interest of civil authorities for future research are not seem to be adequate.

In addition to the active tectonic regime, volcanism played an important role in the geological evolution of Anatolia. Research on volcanism and the natural risk associated with the volcanoes in Turkey is much more complex when compared to active tectonism. In the primary and secondary education of Turkey, it is said that volcanoes of Turkey are all extinct. On the other hand, Global volcanism program indexes 13 volcanoes have had Holocene activities (Table 1.1). Some of these volcanoes have historical records. Thus there are some questions awaiting answer: Can we classify these volcanoes as extinct volcanoes? In a zone where tectonic regime is dominantly active, can the natural risk of the volcanoes be disregarded? Present study concerns with the structural investigation of Mt. Nemrut, one of the youngest volcanoes in Eastern Anatolia and will be focused on the natural risk possibly associated with the volcano.

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Name Status Type Last known

eruption Dating technique

Eruptive characteristics

Kula Holocene Cinder cones Unknown Karapınar

Field Holocene Cinder cones Unknown Mt. Hasan Anthropology Stratovolcano 6200 BC (in or

before) Anthropology Göllüdağ Holocene? Lava dome Unknown

Acıgöl-Nevşehir Anthropology Caldera

2080 BC ± 200

years Anthropology

Explosive eruption Karacadağ Holocene Shield volcano Unknown

Mt. Erciyes Holocene? Stratovolcano 253 AD (in or before) Eruption is UNCERTAIN Flank (excentric) vent, Explosive eruption (?)

Mt. Süphan Holocene Stratovolcano 8050 BC (?) Tephrochronology

Flank (excentric) vent, Explosive eruption (?) Girekol Holocene Unknown

Mt. Nemrut Historical Stratovolcano 1692 Apr 13 Eruption is UNCERTAIN

Explosive eruption (?) Mt. Tendürek Historical Shield volcano 1855 Historical Records Explosive

eruption Mt. Ağrı

(Ararat) Historical Stratovolcano 1840 Jul 2 Historical Records

Explosive eruption, Mudflow(s) (lahars)... Kars Plateau Holocene? Volcanic field Unknown

Table 1.1. Young volcanism in Turkey. Data from Global Volcanism Program (GVP: Simkin and Siebert, 2002-). Locations of these volcanoes can be found on Figure 1.2.

1.1. Tectonics and Volcanism in Anatolia

Complete demise of the Paleotethyan Ocean initiated the continental rifting in the present-day Mediterranean region in the Late Triassic and resulted in the opening of a Mesozoic Neotethyan Ocean (Şengör and Yılmaz, 1981). This rifting ceased during the Middle Jurassic with the development of a passive margin through the south of Cyprus (Garfunkel, 1988) while complex processes of terrain accretion and new continental crustal build-up started to the north (Şengör and Yılmaz, 1981; Şengör and Natal’in, 1996). Convergence between African and Eurasian plates, which began in the Late Cretaceous (Şengör and Yılmaz, 1981) resulted in the progressive closure of these ocean basins and amalgamation of the surrounding continental fragments (Bozkurt, 2001).

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These relative plate motions resulted in the closure of the northern branch of Neotethyan Ocean and suturing in Anatolia during Paleogene and Early Miocene (Şengör and Yılmaz, 1981). Northward subduction of the African plate along Cyprean arc begun during the Early Miocene and during the middle Miocene, Arabia was separated from Africa along the left-lateral Dead Sea fault zone (e.g. Le Pichon and Gaulier, 1988; Fig. 1.1). Accompanying relative motion between Arabian and Eurasian plates formed Bitlis suture zone (Yürür and Chorowicz, 1998).

Figure 1.1. Simplified tectonic map of Turkey showing major neotectonic structures and

neotectonic provinces (after Şengör et al., 1985; Barka, 1992; Bozkurt, 2001). NAFZ – North Anatolian Fault Zone, EAFZ – East Anatolian Fault Zone, DSFZ – Dead Sea Fault Zone, NEAFZ – Northeast Anatolian Fault Zone. Data for fault and plate slip rates indicated in boxes are from (1) Şengör and Yılmaz, 1981; (2) Straub and Kahle, 1995; (3) Reilinger et al., 1997; (4) Barka and Reilinger, 1997; (5) Westaway, 1994; (6) Barka, 1992; (7) Oral et al., 1995; (8) Reilinger et al., 2006. Datum: WGS84.

Later tectonic regime totally differs from the previous history of Anatolia. African-Arabian-Eurasian collision resulted in the formation of four neotectonic structural features in Anatolia: North Anatolian Fault Zone (NAFZ), East Anatolian Fault Zone (EAFZ), Northeast Anatolian fault (NEAFZ) zone and Bitlis suture zone (Fig. 1.1; McKenzie, 1970, 1972; Dewey and Şengör, 1979; McKenzie and Yılmaz, 1991; Bozkurt, 2001) dividing Anatolia into four main neotectonic provinces namely Western Anatolian extensional province, Central Anatolian “Ova” (plain)

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province, North Anatolian province and East Anatolian contractional province (Şengör et al., 1985; Bozkurt, 2001).

The Anatolian plate is clearly “escaping” westward (Fig. 1.1) into the western Mediterranean oceanic tract, where its motion relative to Africa, is taken up by subduction at the Aegean Trench (Dewey and Şengör, 1979). Neotectonic movement of Anatolian plate has been observed by many researchers (e.g. Straub and Kahle, 1995; Reilinger et al., 1997; Barka and Reilinger, 1997; McClusky et al., 2000; Reilinger et al., 2006) and simply shown in Figure 1.1. The GPS-derived velocities for the interaction zone of the Arabian, African (Nubian, Somalian), and Eurasian plates indicates counterclockwise rotation of a broad area of the Earth’s surface including the Arabian plate, adjacent parts of the Zagros and central Iran, Turkey, and the Aegean/Peloponnesus relative to Eurasia at rates in the range of 20-30 mm/yr (Reilinger et al., 2006). This relatively rapid motion occurs within the framework of the slow-moving (~5 mm/yr relative motions) Eurasian, Nubian, and Somalian plates (Reilinger et al., 2006).

Within the frame of neotectonics of Anatolia, volcanism played an important role. Volcanics cover an area more than 250,000 km2 which constitutes approximately ¼ of the total area of Turkey (Fig. 1.2). Volcanism in Northern Turkey is represented by older series. Quaternary and Neogene volcanism appears widely in the western, central and eastern Turkey. In western Anatolia, volcanic activity began during the Late Oligocene – Early Miocene compressional regime, represented by a widespread suite of andesitic and dacitic calc-alkaline rocks. Then the change from N-S compression to N-S stretching in the Middle Miocene was accompanied by a gradual transition to alkali basaltic volcanism (Yılmaz, 1990). In the Eastern Anatolia, volcanic activity began in the Late Miocene to Pliocene and continued into historical times. There is still a lack of knowledge about the Eastern Anatolian volcanism.

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Figure 1.2. Layout of volcanic rocks in Turkey. Data from: MTA, 1964; Aydar, 1992; Pawlewicz et al., 1997. Percentages refer the area covered by the related color in the map relative to the area of Turkey. AV: Ağrı volcano, AcV: Acıgöl caldera complex, EV: Erciyes volcano, GiV: Girekol volcano, GöV: Göllüdağ complex, HaV: Hasan volcano, KaV: Karacadağ volcano, KPT: Kars plateau tuffs, KpV: Karapınar field, KuV: Kula volcanics, NV: Nemrut volcano, SV: Süphan volcano, TV: Tendürek volcano. Datum: WGS84.

Widely-known volcanoes of Eastern Anatolia are Ağrı, Süphan, Tendürek and Nemrut volcanoes. As well, they are not the only ones; Eastern Anatolia hosts a lot of volcanoes which are almost unknown (Fig. 1.3). Although there is some research on the eastern Anatolian volcanism, they represent a general approach in the context of petrology, geology and regional tectonics. Yet, there is limited research on the most well-known volcanoes in the region.

Eastern Anatolia is an area of special interest from the point of view of global tectonics (Innocenti et al., 1976) and the tectonic regime is certainly in close relation with the volcanism. Movement of Arabian plate (15 – 25 mm/yr: Reilinger et al., 1997; Barka and Reilinger, 1997; Oral et al., 1995; McClusky et al., 2000; Reilinger et al., 2006) relative to the Eurasian plate (Fig. 1.1) is fronted in the Caucasian belt forming the thrust zone. Relaxation of the Anatolian plate is accompanied by counter-clockwise rotation of the Anatolia in its western part with the help of NAFZ. On the contrary, although there is a general agreement for the

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initiation and evaluation of the compressional regime in the eastern Anatolia, there are different approaches on the consequences of the compression and on the nature of dynamics of the Eastern Anatolian crust (Şengör and Kidd, 1979; Yılmaz, 1990; McClusky et al., 2000; Bozkurt, 2001; Koçyiğit et al., 2001; Şengör et al., 2003; Dhont and Chorowicz, 2005; Angus et al., 2006; Facenna et al., 2006; Reilinger et al., 2006).

Figure 1.3. Main volcanic centers in the eastern Anatolia. AV: Ağrı volcano, AçV: Akça volcano,

AkV: Akdoğan caldera, BV: Bilican volcano, BiV: Bingöl caldera, BoV: Bozdağ caldera, ÇV: Çıplak (Topdağı) volcano, GV: Gel volcano, GiV: Girekol volcano, HV: Hayal volcano, KV: Kandil volcano, KaV: Karacadağ volcano, KPV: Kargıpazarı volcanoes, KPT: Kars Plateau tuffs, MV: Meydan caldera, NV: Nemrut caldera, SV: Süphan volcano, TV: Tendürek volcano, YV: Yıllık volcano, ZV: Zor volcano. Colors of the volcanic rocks are as in Figure 1.2.

Eastern Anatolian tectonics may be discussed in terms of three main structural elements (Figs. 1.3 and 1.4; Koçyiğit et al., 2001; Bozkurt, 2001; Dhont and Chorowicz, 2005), these are; (1) NW-SE and NE-SW trending dextral and sinistral active strike-slip faults, (2) N-S, NNW-SSE and NNE-SSW trending/elongated fissures and/or Plio-Quaternary volcanoes (Fig. 1.4), and (3) undeformed basins related to strike-slip and/or trust faults which are filled with Plio-Quaternary volcano-sedimentary sequences (Fig. 1.3).

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Figure 1.4. Simplified geological map showing major compressional and extensional structures

(from Koçyiğit et al., 2001). Locations: BY. Bayburt, E. Erzurum, Er. Erevan, K. Kağızman, and Sp. Spitak; Volcanoes: AE. Allahuekber volcano, AG. Alagöz volcano, AK. Akbabadağ volcano, ALD. Aladağlar volcano, Ar. Ağrı volcano, AS. line of Aboul–Samsar volcanoes, AZ. Azizan volcano, BD. Böğütlüdağ volcano, Ç. Çatak volcano and fissure, GR. Girekol volcano, GU. line of Guegam volcanoes, J. line of Javakheti volcanoes, KP. line of Kargapazarı volcanoes, KRD. Karacadağ volcano, N. Nemrut volcano and fissure, S. Süphan volcano, and V. line of Vardinis volcanoes; AAB. Basins: Aktaş-Akhalkalaki basin, AB. Ağrı basin, ARB. Ardahan basin, ASB. Ahaltsikhe basin, BYB. Bayburt basin, HB. Horasan basin, HKB. Hasankale basin, KB. Karasu basin, KÇB. Karaçoban basin, KLB. Kelkit basin, KTB. Kağızman–Tuzluca basin, MB. Muş basin, and TB. Tercan basin; Fault zones: AÇFZ. Akdağ–Çayırlı fault zone, BF. Başkale fault, BGF. Balıkgölü fault, BKF. Borjomi–Kasbeg fault, ÇF. Çaldıran fault, ÇDS. Çobandede fault segment, ÇFZ. Çobandede fault zone, DF. Doğubeyazıt fault, DFZ. Dumlu fault zone, EF. Ercis¸ fault, EFZ. Erevan fault zone, ESFZ. East Samegrelo fault zone, HF. Horasan fault, HTF. Hasantimur Lake fault, IF. Iğdır fault, KBF. Kavakbaşı fault, KÇFZ. Kelkit–Çoruh fault zone, KGF. Kağızman fault, KLS. Kelkit fault segment, KRF. Karçal reverse fault, KS. Kura fault segment, LDF. Leninakan–Digor fault, MF. Malazgirt fault, MTGC. Master thrust of Great Caucasus, NATF. North Adjara–Trialetian thrust fault zone, PS. Posof fault segment, PSFZ. Pambak–Seven fault zone, SATFZ. South Adjara–Trialetian fault zone, SF. Süphan fault, SMF. Salmas fault, TAFZ. Tercan–Aşkale fault zone, TF. Tutak fault, WSF. West Samegrelo fault, and YFZ. Yüksekova fault zone.

Two systems of strike-slip faults occur in the east Anatolian Plateau: (a) NW-SE-trending dextral strike-slip faults paralleling the North Anatolian Fault Zone (NAFZ,

Figs. 1.1 and 1.4) with the same sense of motion; (b) NE-SW trending sinistral strike-slip faults paralleling the East Anatolian Fault Zone (EAFZ) with the same sense of motion (Koçyiğit et al., 2001; Bozkurt, 2001). These two fault systems are same in age (Late Pliocene) and they are connected with stress field linked to the

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N-S directed intra-continental convergence between the Eurasian and Arabian plates (Koçyiğit et al., 2001). Numerous strike-slip basins occur in the east Anatolian plateau (Fig. 1.4). They can be categorized, based on their age and type, into two groups: (1) Oligo-Miocene inverted basins or superimposed basins, and (2) newly formed pure strike-slip basins resulted from the geometric discontinuities including step-overs, bends and bifurcations along through going fault segments (Koçyiğit et al., 2001).

The third group of structures characterizing the east Anatolian plateau is the fissures, alignment of volcanic cones (Koçyiğit et al., 2001). Elongated volcanic edifices or craters and clusters of aligned vents are rooted on tension fractures and extensional features that are related to the step-over and horsetail geometries of the strike-slip faults (Adıyaman, 1998). Fissures or local extensional normal faults are well exposed at the summits of large isolated-to-composite strato-volcanoes of Plio-Quaternary age (Koçyiğit et al., 2001).

Koçyiğit et al. (2001), notes these fissures as isolated and NNW-SSW-trending single crack, or a zone of cracks ranging from 30 m to 2 km in width and 400 m to 50 km in length. Dhont and Chorowicz (2005), defines a mean value of N05ºE-trending fissures and extensions on the eastern Anatolian volcanoes (Fig. 1.5). On the other hand, different trends of the extensional features on the large edifices such as Mount Ağrı, Mount Aragat, Mt. Süphan are evident. Structure of eastern Anatolian volcanoes must be studied in detail for a more comprehensive approach in the context of plate dynamics.

The N-S-directed compressional-contractional tectonic regime and related structures (folds, thrust-to-reverse faults and ramp basins) are prominent in the north (Great Caucasus and the Transcaucasus), while the compressional-extensional tectonic regime related structures (both the sinistral and dextral strike-slip faults, various strike-strike-slip basins and N-S trending fissures) become prominent in the south (the Lesser Caucasus and east Anatolian plateau, Figure 1.4; Koçyiğit

et al., 2001). Tectonic evolution of east Anatolian plateau is schematically synthesized in Figure 1.5.

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Figure 1.5. Schematic illustration of tectonic evolution of eastern Anatolia (Data from: Şengör and Kidd, 1979; Koçyiğit, 2001; Bozkurt, 2001, *Dhont and Chrowicz, 2006). Lines in figures: strike-slip faults, line with triangles: thrusting, red ellipse: elongation of volcanoes and volcanic fissures, black arrows: compression, white arrows: extension.

1.2. Nemrut Caldera

Plio-Quaternary volcanism played an important role in the present morphology of Eastern Anatolia. Mount Nemrut, situated to the western tip of Lake Van is one of the main volcanic centers in the region (Fig. 1.6), with a spectacular summit caldera 8.5 x 7 km in diameter (Fig. 1.7, Aydar et al., 2003; Ulusoy et al., 2008). Within 100 km radius, there are seven other volcanoes; these are, Süphan, Bilican, Akdoğan, Bingöl, Çıplak, Bozdağ and Meydan volcanoes (Fig. 1.6).

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Figure 1.6. Map showing the study area and the volcanoes around the study area. Datum:

European 1950 (UTM).

Nemrut caldera is situated just north of Bitlis-Zagros suture zone, close to the Bitlis edge (Figs. 1.1, 1.4 and 1.6). The Bitlis Suture is a complex continent-continent and continent-ocean collisional boundary that lies north of fold-and-thrust belt of the Arabian platform and extends from southeastern Turkey to the Zagros Mountains in Iran (Şengör and Yılmaz, 1981; Bozkurt, 2001 and references therein). Bitlis suture closed in the Eocene. This closure was then followed by prolonged convergence that involved distributed shortening all over the place, and then the strike-slip fault zones (NAFZ and MOFZ) came into being at <5 Ma, then the geometry changed at <3 Ma when the EAFZ developed (Bozkurt, 2001).

Muş basin with 10-18 km width and ~92 km length is the most important structural feature at the western side of the volcano. It corresponds to the deformed and dissected remnant of the WNW-ESE-trending Oligo-Miocene Muş-Van basin

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located at the northern foot of the Bitlis suture zone (Koçyiğit et al., 2001). Although the Muş basin seems to still retain its earlier nature of ramp basin, its northern margin-bounding reverse fault has a considerable dextral strike-slip component, implying an inversion in the nature of tectonic regime in the Pliocene (Koçyiğit et al., 2001). On the other hand, Dhont and Chorowics (2006) define this basin as a half-ramp basin and indicate the northern boundary fault as a transtensional dextral oblique-slip fault.

Lake Van, by its volume of 576 km3 is the fourth largest terminal lake and the largest soda lake on Earth (Landmann et al., 1996). The surface area amounts to 3522 km2; its maximum depth reaches 460 m (Landmann et al., 1996). The water of the Lake is highly alkaline, with a pH of 9.8, and brackish with a salinity of 22 ‰ (Landmann and Kempe, 2005). Although the faults forming the Muş basin are WNW-ESE-trending, structural features in the Lake Van seem to be mainly directed in ENE-WSW and NNW-SSE (Fig. 1.6). Both this slight reversal in the structural alignments and location of Nemrut volcano at the center of this reversal is important in the context of regional tectonics.

Close to the Nemrut Caldera, İncekaya tuff cone, and Mazik and Girekol domes (Fig. 1.7) may be regarded as in the system of Nemrut volcanism. Nemrut volcano is in the vicinity of Bitlis city and closest populated towns are Tatvan, Ahlat and Güroymak towns (Fig. 1.7 and Table 1.2). Kirkor domes to the south and Nemrutbaşı cone to the north of the caldera are important parasitic volcanic features around the volcano (Fig. 1.7). There are five intra-caldera lakes. Three small lakes are seasonal, while western half of the caldera is filled by a fresh water. Near the northern rim, another lake with hot springs (~60º) is present where fumarolic activity can also be observed.

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Figure 1.7. Study area, main roads, populated places, and important hydrogeologic, topographic

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1.3. Previous studies

Previous research on Nemrut volcano can not be disregarded amongst other poorly studied east Anatolian volcanoes. Latest research on east Anatolian volcanoes mainly focused on this volcano. Özpeker (1973a) studied petrogenesis of the volcano and presented a large scale geologic map. Güner (1984) presents a study for a general geological/volcanological approach on the Nemrut caldera. Research of Atasoy et al. (1988) is one of the extensive works carried out on the volcano focusing mainly geothermal energy potential of the volcano, also deals with volcanology and petrology of the volcano. Ünlü and Can (1983) also discussed the geothermal energy potential of the volcano. Bal (1986) presented the results of a magnetic etude aiming to investigate the geothermal potential of the volcano. Yılmaz et al. (1998) presents a general geological and petrologic state of the volcano together with other well-known volcanoes of eastern Turkey. More recently, research was focused on the petrology of Nemrut volcano (Özdemir et al., 2006, Çubukçu et al., 2007); lately Çubukçu (2008) presented a detailed and extensive research on the context of petrography and petrology. Karaoğlu et al. (2005), deals with the caldera forming eruptions and their stratigraphy. On the other hand, there is no research on the structure, structural evolution, and potential activity of the volcano. Natural risk of this potentially active volcano was also never studied before. Ersoy et al. (2006), proposed a qualitative method for qualitative textural discrimination of volcanic ashes, and applied the method to phreato-magmatic ash samples from Nemrut caldera. Although they are limited, there are also historical and mythological bibliography and researches (Şerefhan, 1597; Karakhanian et al., 2002, 2006; Gadjimuradov and Schmoeckel, 2005) that should be mentioned. Historical inscriptions and myths are complied and shortly summarized in Appendix A.

Results of this study were previously presented in (and submitted to) international peer reviewed journals and international conference proceedings (articles: Aydar et al., 2003 (Appendix B); Ulusoy et al., 2008; Ulusoy et al., submitted; proceedings: Ulusoy et al., 2006a, 2006b, 2007).

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1.4. Structural Approach and Natural Risk of the Nemrut Caldera

Nemrut volcano is located on a highly active tectonic zone: high magnitude seismic events have been reported (29.03.1907, M: 5; 27.01.1913, M: 5; 14.02.1915, M: 6; 03.11.1997, M: 5; 30.05.1881 (data from Boğaziçi University, Kandilli Observatory and Earthquake Research Institute, National center of earthquake monitoring.); 18.05.1881, M: 6.7 (Karakhanian et al., 2002) within a 30 km radius of the volcano during the last century (Ulusoy et al., 2008). One of the most active tectonic zones in the world; Karlıova triple junction (Fig. 1.6) is located 125 km NW of the volcano, and Bitlis suture zone (Fig. 1.7) is 16 km south of the volcano. Structural evolution of the volcano in this active tectonic regime, its relation with the volcanic evolution and its role in the past and potential future activity forms one of the main aims of this study.

Latest works documented that Nemrut volcano witnessed volcanic activity in the last millennium (Aydar et al., 2003; Ulusoy et al., 2008; Karakhanian et al., 2002; 2006). Wisps of smoke and hot springs can be found inside the caldera, hot springs also appear around the Mazik and Girekol domes at the eastern flank of the volcano. The present active tectonic regime, historical eruptions, occurrence of mantle-derived magmatic gases (Nagao et al., 1989; Güleç et al., 2002), the fumarole and hydrothermal activities on the volcano make Nemrut Volcano a real danger for its vicinity (Aydar et al., 2002; Ulusoy, et al., 2006b). The population in the vicinity of the volcano (~135,000), especially in the nearby towns is significantly important (Table 1.2 and Fig. 1.7). Besides, Bitlis city grows to the north, towards the volcano. Nagao et al. (1998), and Feraud and Özkocak (1993), previously pointed out that Nemrut volcano may be potentially active. As well as other Anatolian volcanoes, there is a big gap on the research on the activity of Nemrut volcano. Second aim of this work is to identify the previous and the current activity of the volcano and their structural relationships.

1990 2000 2007

Total City Village Total City Village Total City Village BİTLİS City center 68 132 38 130 30 002 65 169 44 923 20 246 Tatvan 81 992 54 071 27 921 84 276 66 748 17 528 Güroymak 37 030 16 613 20 417 48 118 22 521 25 597 Ahlat 34 217 16 742 17 475 52 814 34 787 18 027 Total 330 115 144 029 186 086 388 678 219 511 169 167 327 886 179 260 148 626

Table 1.2. Demographical data from 1990, 2000 and 2007 consensus for the vicinity of the Nemrut volcano (Data from Turkish Statistical Institute).

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1.4.1. Methodology

Wide variety of methodological approaches was used to introduce the structural evolution of the volcano and the associated natural risk. A comprehensive literature study was carried out before and during the study. Needful data found in the literature studies were complied and necessary data were digitized to reinforce the study.

Available useful digital data (such as SRTM elevation data, Landsat ETM+ imagery, GIS data, Seismic data, and others) provided for free use in the World Wide Web were collected, adopted and integrated to the database of this study. Earth Science Data Interface of Global Land Cover Facility, USGS open file reports and Boğaziçi University, Kandilli Observatory and Earthquake Research Institute, National center of earthquake monitoring are some of these databases. Geological, volcanological and geophysical field studies constitute an integral part of this study. To reveal the volcanological and volcano-structural evolution, geological field surveys were focused on the structural context. General stratigraphy of the volcanic rocks and particularly caldera forming eruptions and their products were studied in detail.

Geophysical surveys constitute one of the fundamental parts of this study. Self-potential surveys were applied, not only to reveal the current hydrothermal activity of the volcano, but also to reveal the hydrothermal fluid circulation and its relation with the internal structure of the caldera. Research on hydrothermal condition of the caldera was supported by remote sensing approaches. Both diurnal imagery and night-time TIR imagery were used to analyze the hydrothermal background and status of the volcano.

Monitoring of the volcano was essential to constitute a general idea about the current activity state of the volcano and to make a quantitative approach on the natural risk potential of the volcano. For the first time, a seismological study was carried out on a volcano in Turkey. A small seismological network was installed around the caldera to monitor the volcano-seismic activity. Various problems were faced during the data transfer, maintenance, and analysis processes; these problems also gathered knowledge and experience in construction and maintenance of such systems. Additionally, for the monitoring purpose, four thermo-data-loggers have been recently installed on the volcano.

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When necessary, computer codes were written to support, fasten, automate and facilitate the data processing and handling. Three codes were written to for analyzing and handling the seismic network. Another code (Ulusoy et al., submitted) using a new method was written for the image based retrieval of altitude and aspect effects on night time TIR imagery.

All the produced data were processed in computer environment and appended in the GIS environment. Extensive GIS database constructed for Nemrut volcano significantly aided and fastened the study and will facilitate future research on the volcano.

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Chapter 02

Geology

and

Caldera forming eruptions

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2. Geology, Caldera forming eruptions

Beyond the geophysical, morphological and structural studies, in order to comprehend the evolution of the Nemrut volcano, a detailed geological study was carried out. This section discusses the Quaternary Nemrut volcanism under two subheadings as pre-caldera and post-caldera volcanism. With a special emphasis, pyroclastic stratigraphy and characteristics of pyroclastic deposits are discussed in detail for a better understanding of the formation stage of the caldera. Other then the Quaternary Nemrut volcanism, pre-volcanic basement is summarized shortly. Recent activity will be summarized at the end of the chapter.

Geological Map

During the study, a detailed geological map of the volcano has been elaborated (Fig. 2.1). Geological rock names were defined according to the extensive petrological work of Çubukçu (2008). We benefited from geological maps of Güner (1984) and Atasoy et al. (1988), especially at the northernmost section of our map area, but our map offers major additions and changes to previous maps. It should be appropriate to discuss these changes here, because important errors have been pointed out on previous maps.

Former geological maps of the volcano were presented by Özpeker (1973a), Güner (1984) and Atasoy et al. (1988). These maps were followed by maps of Yılmaz et al. (1998) and Karaoğlu et al. (2005) with no major changes. Contrarily, they followed the same mistakes. Aydar et al. (2003) presented a modified version of Yılmaz et al. (1998)’s map. Özdemir et al (2006) used the same map with Karaoğlu et al. (2005).

In all of these maps, there is a major discrepancy on the layout and the type of the pyroclastic units. Özpeker (1973a) mapped a pyroclastic unit encircling the topographic rim of the volcano, and combines this unit with the products of intra-caldera maar eruptions cresting the eastern intra-caldera wall. He defined one ignimbrite unit on the flanks of the caldera. Yılmaz et al. (1998) presented the same map with Güner (1984). Güner (1984) was the first who mapped two different caldera forming ignimbrite series. Atasoy et al. (1988) also mapped these two ignimbrite series; they updated the boundary between the two ignimbrites mostly at northern and northwestern flanks of the volcano. But in their map, they did not completely separate these two series; they named the former as

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“Crystalline welded tuff + pumice” and the latter as “Crystalline welded tuff”. However, in their report, Atasoy et al. (1988) clearly indicated that they identified two different ignimbrite flows. Karaoğlu et al. (2005) surprisingly removed the second ignimbrite series, disregarded previous definitions and related the collapse of the caldera with only one ignimbrite series. Additionally, they used approximately the same boundaries for the upper ignimbrite series defined by Güner (1984) and Atasoy et al. (1988), but changed the unit to “plinian fall deposits”. In our fieldwork, we defined two ignimbrite series leaded to the collapse of the caldera and mapped them carefully (Fig. 2.1, Nemrut ignimbrites and Kantaşı ignimbrites). For the first time, we defined and mapped two other ignimbrite units older than the products of caldera forming eruptions (Fig. 2.1, Tuğ

ignimbrite and Yasintepe ignimbrite).

The second discrepancy initiated with the map of Güner (1984) and followed by Karaoğlu et al. (2005) without acknowledging his work. At the northern flank of the caldera and around the rift zone, Güner (1984) defined many lava flows and named them as “scoria flows”. According to us, this was a considerably fundamental error. In the maps of Güner (1984), Yılmaz et al. (1998) and Karaoğlu et al. (2005), these “scoria flows” are originated from the ridge forming the Nemrut rift zone between Kantaşı hill (Fig. 2.1) and northern rim of caldera and flowed along eastern and western side of this ridge up to ~4 km distance. We particularly want to indicate that there are no “scoria flows” in this area. Whole area is covered with the Kantaşı ignimbrites. Latest activity of Nemrut volcano generated along the rift zone, and few comenditic and basalt flows originated from the rift flowed to east and western side of the rift. Boundaries of these few lava flows are very clear and the flows extended to a maximum distance of about 1.5 km. They are lying upon the Kantaşı ignimbrites. In the close vicinity of the rift zone there are ballistic ejecta and there are dispersed basalts of aa-like lava flows, but these vesicular basalts are limited and the dispersion is not dense. These materials and highly welded Kantaşı ignimbrite were most probably confused with “scoria flows” and caused the error in the maps of Güner (1984), Yılmaz et al. (1998) and Karaoğlu et al. (2005). This error was corrected totally in Atasoy et al. (1988) and partially in Aydar et al. (2003).

The third major error was introduced to the literature with the maps of Karaoğlu et al. (2005) and Özdemir et al. (2006). They define monzonitic intrusions exposed in

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topographic lows within the caldera. Çubukçu et al. (2006) discussed this error in detail. They have neither given any petrographical/mineralogical description nor geochemical analysis of this unit (Çubukçu et al., 2006). In addition, the area mapped (~0.4 km2) as monzonite is within intra-caldera maars whose post-caldera products contain abundant holocrystalline fragments (Çubukçu et al., 2006). These intra-caldera maars are among the centers of post-caldera phreatic/phreatomagmatic activities (Çubukçu et al., 2006). Their basements are filled with post-eruption deposits while their walls are subjected to intense hydrothermal alteration (Çubukçu et al., 2006). This error was checked again in our later fieldworks and no monzonitic bodies and rocks defined in Karaoğlu et al. (2005) and Özdemir et al (2006) were observed.

We introduced a map purified from these errors and removing the discrepancy on the definition of pyroclastic units.

2.1. Pre-volcanic basement

Pre-volcanic basement rocks can be divided into three main groups as Bitlis metamorphics, Çatak ophiolites, and Tertiary sediments (Ahlat formation). The stratigraphy of the basement rocks in the Nemrut area is fairly well known (Gunderson, 1988). However, the basement rocks have been structurally disrupted several times since the Mesozoic (Gunderson, 1988). In the vicinity of Nemrut, all of the pre-volcanic basement rocks are cut by the faults which separate Muş basin from Bitlis metamorphics on the south and from Tertiary sedimentary rocks on the north (Gunderson, 1988).

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2.1.1. Bitlis Metamorphics

The oldest crustal rocks underlying Nemrut are the Precambrian to Mesozoic Bitlis Metamorphics (Güner, 1984; Gunderson, 1988). They cropped out along the steep mountains at the southern margin of Muş basin (Atasoy et al., 1988), ~15 km south of the Nemrut (Fig. 2.1). The Bitlis Massif forms a part of the Tethyan suture zone that was assembled during Late Mesozoic – Early Cenozoic time (Yılmaz et al., 1993; Robertson, 1998). It is a regional-scale allochthonous unit with a high-grade metamorphic basement and a lower-high-grade cover sequence (Göncüoğlu and Turhan, 1984; Ustaömer et al., in press). These metamorphics consist of metapelites, metabasites, amphibolite-biotite gneisses, chlorite-schists, calc-schists, meta-quartzites and recrystalized marbles (Gunderson, 1988; Atasoy et al., 1988). Small granitic plutons and associated granitic dykes intrude the pre-Devonian basement of the Bitlis Massif (Ustaömer et al., in press). The thickness of the units is unknown but presumed to be more than a few kilometers. Historic Urartian stronghold near Tatvan pier was built on recrystalized limestones of Bitlis Metamorphics (Fig. 2.1).

2.1.2. Çatak Ophiolites

Commonly thrust on top of the Bitlis Metamorphics are Cretaceous ophiolitic rocks (Gunderson, 1988; Atasoy et al., 1988), these rocks are usually thrust from north to south, and were emplaced during Eocene and Miocene compressional events (Gunderson, 1988). Çatak ophiolites typically include serpentines, greenstones, cherts, micrites, limestones, and greywackes (Gunderson, 1988). These rocks are cropped out east of Ahlat town, out of our study area. Tertiary sediments are overlain by volcanic products of Nemrut along Yolgözler, Yünören, Sikeftan, Bahçe and Atakır villages, though it is not possible to observe ophiolites. North to the Nemrut volcano, they are probably overlain by volcanic products (Atasoy et al., 1988).

2.1.3. Tertiary Sediments (Ahlat formation)

Tertiary sedimentary rocks overlie Cretaceous ophiolitic rocks unconformably (Gunderson, 1988). The sedimentary rocks were deposited in small, usually E-W-trending elongated basins that were opened between the Eocene and Miocene (Gunderson, 1988). The sedimentary sequences typically include sandstones,

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mudstones, carbonates, lacustrine and coarse-grained fluvial deposits (Gunderson, 1988). Tertiary sedimentary rocks outcrop to the west and northwest of Nemrut volcano and at the east of the Ahlat Town. Ahlat formation is represented by Eocene and Oligocene conglomerates and sandstones, Miocene limestones, and Pliocene lacustrine deposits (Güner, 1984; Gunderson, 1988; Atasoy et al., 1988).

2.2. Volcanism in the vicinity of Mt. Nemrut

In the near vicinity of the Nemrut volcano, there are four main volcanic centers: Süphan volcano, Bilican volcano, Kolango dome, and İncekaya Tuff Cone. A short note on these volcanoes is essential to define the structural state of the region and the relationship between the products of these volcanoes and Nemrut volcano. İncekaya Tuff cone is thought to be activated during the pre-caldera activity of Nemrut volcano. Our remarks on the İncekaya volcanism will be discussed below (see section 2.2.3).

2.2.1. Süphan Volcano

Süphan volcano is situated 60 km northeast of Mt. Nemrut and 15 km north of Lake Van (Fig. 1.6). It is the second highest mountain of Turkey with an elevation of 4158 m. It culminates on the well known (e.g. Dhont and Chorowicz, 2006) NE-SW directed sinistral Süphan fault. Quaternary volcanism (Notsu et al., 1995) of Mt. Süphan is compositionally defined as mildly sub-alkaline (Yılmaz et al. 1998). In the study area, ignimbrite units of Süphan volcano outcrops around Ahlat town commonly in the valleys (Fig. 2.1). In the Ahlat town, the historic caves (Fig. 2.1) were built into these ignimbrites (Fig. 2.2a). The ignimbrite unit is consolidated, beige in color, bears white pumices and the thickness of the unit reaches 13 m. Another unit of Süphan ignimbrites appears closer to the Lake Van; it is reddish burgundy in color and it bears coarse prismatic quartz crystals in pumices. This unit is overlain by lacustrine sediments. Pyroclastics of Süphan volcano outcrops at the northeastern side of the Karnıç stream which is bounding Lake Nazik to Lake Van. Both ignimbrite units of Süphan volcano are covered by the fall-back units of Nemrut ignimbrite series. Near the historic ruins (Fig. 2.1) in the Ahlat town, the thickness of these fall-back units reaches up to ~17 m (Fig. 2.2b).

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Figure 2.2. Pyroclastic outcrops in the Ahlat town. a) Historic caves in the upper ignimbrite unit of

Süphan volcano. b) Fall-back units of Nemrut ignimbrites overlying Süphan ignimbrites.

2.2.2. Bilican volcano and Kolango dome

Bilican volcano lies 38 km north of Mt. Nemrut (Fig. 1.6); highest peak of the edifice is 2754 m. Other than the morpho-structural definition made by Adıyaman et al. (1998), the information about Bilican is limited. Calc-alkaline Bilican volcano is a volcanic edifice rooted on a tension fracture (Adıyaman et al., 1998). The tension fracture has about 12 km length and at the largest point, its width is about 3 km (Fig. 1.6). At north of the edifice, there are five adjacent cones forming a linear cluster trending N12ºE (Adıyaman et al., 1998). Several small volcanic cones are found near the Bilican volcano (Adıyaman et al., 1998). To the east, they form two linear clusters parallel to the main one. To the west, cones are not adjacent to each other but they are still aligned in the same direction. To the south, no linear cluster can be identified but some of the edifices are elongated approximately in the N–S direction (Adıyaman et al., 1998).

Kolango dome is situated 26 km north of Nemrut volcano and 10 km SSW of Bilican volcano at the western shore of Nazik Lake (Fig. 2.1). There is a lack of data about this volcano; the Kolango dome could be included to the Bilican system. Its summit culminates at 2321 m. A NW-SE-trending lineament passing through the summit of the dome is evident on the satellite images. Moreover, there is also a WNW-ESE-trending rift zone along the ridge westbound of Kolango dome. Pınardüzü hills (Fig. 2.1) form the northern ridge of the rift zone which is about 6 km long and reaches to ~800 m width. The WNW-ESE-trending rift zone is noteworthy. The volcanism in the eastern Turkey is often said to be rooted on

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N-S-trending fracture/fault systems, but there are many exceptions to this assumption (including Mt. Ağrı, Mt. Süphan, Mt. Girekol, and Mt. Aragat in Armenia).

2.2.3. İncekaya Tuff Cone

İncekaya Tuff Cone is located on the southwestern shore of Lake Van, 25 km SE of Nemrut volcano; it rises 387 m from the Lake level (Figs. 2.1 and 2.3). Bitlis Metamorfics constitute its basement (Fig. 2.1). Özpeker (1973a), and Güner (1984) defined the İncekaya system as a caldera but, both its morphology and products clearly depict that İncekaya is a tuff cone. There is a relatively small maar (İncekaya maar) forming the NE end of the tuff cone at Zin cape (Figs. 2.1 and 2.3). South to the Cone there are six scoria cones and four of them are aligned along an inferred fault (Fig. 2.1). Largest of these cones is situated near Dibekli village and named Dibekli cone. Originating from southern side of the Dibekli dome, a basaltic lava flow is lying (Fig. 2.1).

Figure 2.3. Panoramic photograph of inner view of İncekaya Tuff Cone and İncekaya maar at the

left side.

Surge units of the cone are mainly basaltic in composition (Güner, 1984) and have black, gray and greenish colors. Consolidated accretionary lapilli rich levels, coarse quartz and schist xenoliths belonging to the metamorphic basement are evident in the surge units (Figs. 2.4a, b). Products of the tuff cone, that flown over Lake Van are observed 14 km NNE of the cone, on the İnce cape (Figs. 2.1 and 2.4c). On the hills across the Lake Van that are facing the eastern flanks of İncekaya Tuff Cone, plaques of surge units flown away up to 10 km across the lake, were also observed.

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Figure 2.4. Products of İncekaya Tuff Cone. a) Quartz and b) schist xenoliths in the surge units. c)

Fine-grained units of İncekaya Tuff Cone observed on İnce cape, north of Lake Van.

2.3. Geological evolution of Nemrut volcano

Nemrut volcano started its activity ~1 Ma ago and continued until historical times (Ulusoy et al., 2008; Çubukçu, 2008). Mt. Nemrut exhibits a spectacular summit caldera with dimensions of 8.5 × 7 km. The summit of the caldera rim, Sivri hill, is on the north side and is 2935 m high; the highest point within the caldera is Göl hill (2486 m) located in the eastern part (Figs. 2.1 and 2.28). The western half is filled by a freshwater lake (Nemrut Lake, Fig. 2.1) with a surface area of 12.36 km2, and a smaller lake with hot springs. The altitude of the lake surface is 2247 m. The eastern half of the caldera is filled by pyroclastic deposits of maars, lava domes and flows (Ulusoy et al., 2008).

Since the introductory study of Özpeker (1973a), detailed studies of volcanological evolution of Nemrut caldera were a matter of debate (Çubukçu, 2008). Most of the previous researchers described various evolutionary stages for the volcano without evident structural phase changes or petrological differences. Atasoy et al.

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(1988) divided volcanic evolution of the Nemrut volcano into four stages namely, pre-cone, cone-building, caldera-forming and post-caldera stages. Yılmaz et al. (1989) proposed pre-cone, cone-building, climactic, post-caldera and late phases. However, Yılmaz et al. (1989) also propose the same evolutionary template for other volcanoes of eastern Anatolia (Süphan, Ağrı and Tendürek) which are located on a complex tectonic system and present different petrological characters. Karaoğlu et al. (2005) and Özdemir et al (2006) proposed three evolutionary stages: pre-caldera, post-caldera and late stages. Furthermore, the criteria for such discrimination (i.e. separating the late stage from the post-caldera stage) were unclear, ambiguous and volcanological point of view is lacking (c.f. Çubukçu et al., 2007; Çubukçu, 2008). Aydar et al. (2003) and Çubukçu (2008) suggested two main evolutionary stages intervened by the paroxysmal eruptions leading to the caldera collapse: pre-caldera and post-caldera stages. Moreover, Çubukçu (2008) detailed this volcano-structural discrimination according to the petrological evolution of the volcano. Evolutionary stages of the volcano will be proposed as pre-caldera and post-caldera here (Table 2.1, Fig. 2.5), and the volcano-stratigraphy will be built upon the detailed petrological description of Çubukçu (2008). Evolution of the volcano is also presented as video, produced by modifications of DEM and evolutionary representation of the Geological map of the volcano (e-Appendix-1).

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Stage Sub-stage ID Eruption dates Product Event Method (source)

a1 13.April.1692 ? Eruption of gas and ash Historical (1)

a2 1597 AD Comendite, Basalt Lava fountains and flows Historical (2,3)

a3 1441 AD Comendite, Basalt Lava fountains and flows Historical (1)

a4 657 ± 24 BC Ash Ash eruption varve (4)

a5 787 ± 25 BC Ash Ash eruption varve (4)

a6 4055 ± 60 BC Ash Ash eruption varve (4)

a7 4938 ± 69 BC Ash Ash eruption varve (4)

a8 5242 ± 72 BC Ash Ash eruption varve (4)

a9 7 ± 4 ka (*) Mugearite Lava flow K/Ar (11)

a10 8 ± 3 ka Ash (Comenditic) Phreatic Eruption K/Ar (11)

a11 <10 ka Rhyolite Lava flow K/Ar (7)

a12 9950 ± 141 BC Ash Ash eruption varve (5)

a13 10042 ± 142 BC Ash Ash eruption varve (5)

a14 10111 ± 143 BC Ash Ash eruption varve (5)

a15 10305 ± 145 BC Ash Ash eruption varve (5)

a16 10330 ± 145 BC Ash Ash eruption varve (5)

a17 10356 ± 146 BC Ash Ash eruption varve (5)

a18 11010 ± 166 BC Ash Ash eruption varve (5)

a19 15 ± 1 ka Comendite Lava flow K/Ar (11)

a20 15 - 19 ka Comendite Lava flow K/Ar (6)

a21 <20 ka Comendite Lava flow K/Ar (7)

a22 24 ± 1 ka Comendite Lava flow K/Ar (7)

a23 <30 ka Comendite Lava flow K/Ar (9)

a24 80 ± 20 ka Olivine basalt Lava flow K/Ar (7)

a25 89 ± 2 ka Comenditic Trachyte Lava flow K/Ar (11)

a26 93 ± 3 ka Comenditic Trachyte Lava flow K/Ar (11)

a27 99 ± 3 ka Pantellerite Lava flow K/Ar (11)

a28 100 ± 50 ka Mugearite Lava flow K/Ar (7)

a29 158 ± 4 ka Comendite Lava flow K/Ar (11)

a30 242 ± 15 ka Comendite Lava flow K/Ar (6)

a31 263 ± 6 ka Comenditic Trachyte Lava flow K/Ar (11)

a32 272 ka Ignimbrites (?) Ash flow K/Ar (6)

a33 310 ± 100 ka Comendite Lava flow isotope (10)

a34 333 ± 41 ka Comenditic Trachyte Lava flow K/Ar (6)

a35 384 ± 23 ka Pantelleritic Trachyte Lava flow K/Ar (6)

a36 567 ± 23 ka Comendite Lava flow K/Ar (6)

a37 <700 ka Comenditic Trachyte Lava flow K/Ar (8)

a38 1.01 ± 0.04Ma Trachyte Lava flow K/Ar (6)

VII. Intra-caldera Phreatic/ Phreatomag. eruptions, Lava flows and domes P R E C A L D E R A

V. Caldera forming eruptions: Nemrut and Kantaşı ignimbrite series

I. Lava flows II. Lava flows III. Peripheral Doming and Lava flows IV. Lava flows VI. Intra-caldera Lava flows P O S T C A L D E R A VIII. Rift activity (bimodal)

Table 2.1. Historical and older eruptions of the Nemrut volcano. Data source: 1: Karakhanian et al. (2002); 2: Şerefhan (1597); 3: Aydar et al. (2003); 4: Landmann (1996); 5: Landmann et al. (1996) corrected according to Landmann and Kempe (2005); 6: Atasoy et al. (1988); 7: Notsu et al. (1995); 8: Pearce et al. (1990); 9: Ercan et al. (1990); 10: Yılmaz et al. (1998); 11: Çubukçu (2008); (*) indicates the suspicious (Çubukçu, 2008) analysis of Mugearite. Geochemical descriptions of lava flows are taken from Çubukçu (2008). Known locations of dated samples are indicated in Figure 2.5 with reference IDs given in this table (a1-a38).

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Figure 2.5. Stratigraphical evolution of Nemrut Volcano. Roman numerals in the legend reference

the sub-stages given in Table 2.1. Quotations made in white boxes are the known locations of dated samples given in Table 2.1. Geological representation was superimposed on Swiss style hillshade of DEM. Projection: UTM, European Datum 1950.

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2.3.1. Pre-caldera activity

Pre-caldera stage of Nemrut volcanism extends from 1.0 Ma to 80 ka. This construction period can be divided into five sub-stages (Table 2.1, Fig. 2.5: I, II, III, IV, V); two initial sub-stages (I, II) are represented by lava flows and in the third one (III) peripheral doming is associated with lava flows. Lava flows dominated the forth sub-stage (IV), and the final stage (V) corresponds to the pyroclastic activity which lead to the formation of caldera.

Pre-caldera products of Nemrut volcanism, prior to the caldera forming eruptions, are dominantly composed of silica oversaturated trachytes and rhyolites (Çubukçu, 2008). Nevertheless, there are scarce outcrops of basaltic trachyandesites (mugearites) and metaluminous trachytes (Çubukçu, 2008).

It has been suggested that the oldest volcanic products of Nemrut volcanism were fissure basalts (e.g. Özpeker, 1973a; Güner, 1984; Atasoy et al., 1988; Karaoğlu et al., 2005; Özdemir et al., 2006) located in Bitlis Valley, ~45 kilometers south of the Nemrut volcano (Çubukçu, 2008). On the contrary Ercan et al. (1990) proposed a fissural basaltic origin different than Nemrut for these lava flows. Two different ages were obtained from these basalts: <2.5 Ma (Ercan et al. (1990) and 0.79 Ma (Atasoy et al., 1988). Both Çubukçu (2008) and Ercan et al. (1990) suggested that these relatively older Bitlis valley basalts should not be included into the Nemrut volcanic system. These flows most probably belong to an earlier different system.

Sub-stage I (~1.0 Ma – 500 ka)

Nemrut volcanism has been initiated with lava flows represented by metaluminous felsic rocks exposed on the southeastern flanks of the volcano, and continued with, the oldest known peralkaline silicic (Çubukçu, 2008) lavas represented by the samples of Atasoy et al. (1998) and Pearce et al. (1990) taken from the western caldera wall. Lower contacts of these lava flows were not observed in our field studies; consequently this age signifies the lowest limit of temporal space of Nemrut volcanic history (Çubukçu, 2008). These lava flows are about 300 m above the western base of the main cone, it would be viable to state that the volcanism has initiated prior to this activity.

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Sub-stage II (500 – 300 ka)

During sub-stages II and III, volcanic activity seems to be intensified and the central cone was formed (Çubukçu, 2008). The lava flows of the second sub-stage exhibit stratigraphical, thus temporal evolution from trachytic to rhyolitic compositions (Çubukçu, 2008).

Sub-stage III (300 – 200 ka)

The third sub-stage continued to produce rhyolitic and trachytic lava flows. However, peripheral doming marked the third sub-stage, forming Kirkor domes, Yumurtadağ, Fakı, Kalekirana hills and Kale Hills (Figs. 2.1 and 2.6a, b) at the southern flank and Mazik and Girekol domes at the lower western flank of the volcano (Fig. 2.1 and 2.7). The geochemical data and dating are lacking, but stratigraphically domes forming Kayalı, Tavşan and Arizin hills at the northern side of the volcano belong to the same sub-stage. All of these domes are partially covered by the later pyroclastic units.

Kirkor domes (Fig. 2.6a) culminates at 2478 m (western peak) and 2442 m (eastern peak). Domes are comenditic in composition (Çubukçu, 2008) and dated at 242 ± 15 ka (Atasoy et al. 1988) Lavas originating from Kirkor complex have flowed 3 km to the southwest (Figs. 2.1, 2.5 and 2.6a) and formed steep plateaus of ~ 30 m height.

Kale Hills west of the Çekmece village and Kalekirana hill (Figs. 2.1 and 2.6b) are comenditic in composition. These domes are aligned on a fault and as it will be discussed later, the same fault probably separates the caldera into two blocks. Mazik and Girekol domes are at the western flank of the volcano and southeastern end of Muş basin (Figs. 1.6, 1.7, 2.1 and 2.7). Mazik dome is a trachytic lava dome, covered with ignimbrites, that culminates at 1680 m, rising 370 m from Muş basin.

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Figure 2.6. Peripheral domes at the southern flank of Nemrut volcano. a) Kirkor domes and associated lava flows (Comenditic Trachytes) as viewed from

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